WD explains hybrid tech behind Black SSHDs

Ever since WD announced its first SSHDs last month, we've wondered about the specifics behind the firm's hybrid technology. Yesterday, the drive maker shed a little more light on the subject, revealing that its Black SSHDs will use iSSD flash components supplied by SanDisk. The SanDisk iSSD is a standalone SATA device meant for mobile and embedded applications; however, WD provided no details about how the iSSD is integrated into its hybrid drive.

Curious, we spoke to Matt Rutledge, Vice President of WD's client computing group, who was kind enough to explain the hybrid system in greater detail.

Existing hybrid solutions from Seagate and Toshiba manage caching at the drive level, with the firmware determining what gets put in the flash. WD has taken a different path: it uses driver software to control the caching scheme. According to Rutledge, WD wanted its solution to be closer to the user and to the file system. This driver-based approach doesn't have low-level visibility on how data is organized on the SSHD's mechanical platters. However, it receives "hints" from the operating system about whether files contain so-called hot data that should be cached. Data for both read and write requests can be cached.

This scheme sounds reminiscent of Intel's Smart Response Technology, which enables standalone SSDs to work as a cache for mechanical hard drives. While WD has consolidated solid-state and mechanical storage on a single SSHD, those components are still somewhat independent. The SanDisk iSSD has an integrated controller, and so does the mechanical portion of the SSHD. The two controllers are hooked up to a Serial ATA bridge chip that directs the flow of data between them and the host system. What we have here is a highly integrated take on dual-drive caching.

WD's Black SSHDs will be available with 8-24GB of iSSD flash. Interestingly, the NAND has a dual-mode arrangement similar to Seagate's Laptop Thin SSHD. While the bulk of the flash is addressed as MLC NAND, some of it is configured as SLC memory. These "SLC zones" should offer better write performance and endurance than the rest of the NAND. We're waiting to hear back from WD on whether the portion of NAND dedicated to SLC zones is fixed or can be adjusted dynamically.

Although WD's hybrid development started with an off-the-shelf iSSD, we're told the firmware has been heavily customized since. WD's customizations are proprietary, of course. The caching concept is not, and we may see other drive makers introduce products that resemble the Black SSHD. For example, Seagate told us in March that it's working on a new hybrid drive designed to work in conjunction with Intel's caching software.

PC makers using the Black SSHD will be able to choose between WD's proprietary driver and one provided by Intel. The drivers behave similarly, according to Rutledge, but they're not identical. The WD driver was developed in-house and works with both Windows 7 and 8. In an interesting twist, that driver also employs system memory as part of the caching scheme. We'll probably have to wait until the Haswell launch next month to find out exactly what Intel is bringing to the table.

For now, WD is restricting its hybrid tech to mobile offerings. Rutledge argued that, unlike slim notebooks, desktop PCs have plenty of room for dual-drive solutions, which blunts the appeal of integrated SSHDs. He has a point. A dual-drive approach gives you the flexibility to run separate system and storage drives—or to combine the two with software-managed caching. That said, Rutledge conceded that a single hybrid drive might have some appeal for the average desktop user. I suspect we'll see WD offer hybrid desktop drives before too long.

In any event, I'm curious to see how WD's hybrid tech stacks up against the other implementations on the market. Having up to 24GB of flash on a 5-mm, 500GB notebook drive is certainly tantalizing. SanDisk's iSSDs can purportedly hit read and write speeds of 450 and 350MB/s, respectively, and Intel's SRT has already proven that software-based caching can deliver substantial performance gains.

The only potential fly in the ointment is the Black SSHD's spindle speed. Rutledge wouldn't put a number on it, contending that rotational speed doesn't matter for hybrid solutions. That probably means we're looking at something around 5,400 RPM. As we saw with Seagate's Laptop Thin SSHD, such a low spindle speed can really hurt performance in some situations. Let's hope the Black SSHD's larger flash cache can mitigate the damage.